The invention relates to acoustic lens.
A known lens arrangement is shown, for example, in U.S. Pat. No. 4,028,933. A piezoelectric transducer is disposed on one side of a cylindrical sapphire rod, and a spherical concave surface is incorporated in the opposite side. A high-frequency electric field applied to the transducer generates in the sapphire rod a plane acoustic wavefield, which is focused by the spherical concave surface into an adjoining immersion liquid.
The lens arrangement is part of an acoustic microscope. In this connection, an object to be investigated is placed at the acoustic focus. After interaction of the focused acoustic waves with the object (generation of longitudinal waves, bulk waves), acoustic waves proceed from the object, which are caught by the same or another acoustic lens and converted into electrical signals in the piezoelectric transducer. By scanning of the object in the manner of a raster, an image of the object representing the acoustic interaction can be obtained from these electrical signals.
The acoustic waves regularly reflected at the object or those which are transmitted are essentially utilized for acoustic microscopy. However, it is known that acoustic waves which impinge on an object surface at a specific angle dependent on the material (Rayleigh angle .theta..sub.R) excite surface waves in this surface (surface acoustic waves, SAW). Along their path of propagation, the SAW leak in the form of bulk acoustic waves out of the object (leaky SAW). These waves can also be detected and converted into electrical signals. In acoustic microscopy, they are superposed on the regular signal, in particular in the case of focusing on an object region situated below the object surface. By means of particular circuitry, they can also be analyzed separately (cf. West German Patent Application No. P 3,409,929.8).
If the SAW impinge on inhomogeneities in the object surface, the SAW are reflected thereat, so that they change their direction of propagation. The result of this is that leaky waves also appear to an increased extent in this direction. Since the SAW penetrate relatively deep into the object surface, they are now increasingly utilized for the purpose of determining properties of the material of various objects. The particular advantage is that the method which is involved is a non-destructive measurement method, with which quantitative measurements are also possible. For this purpose, it is however necessary to increase the local resolving power and to improve the signal gain.
With regard to the improvement of the SAW measurement method, two problems must essentially be solved. The first problem consists in the generation, in a manner as efficient as possible, of the SAW in the surface of the material to be investigated, which as a rule is not piezoelectric. The second problem consists in focusing the generated SAW on the smallest possible spot size.
Specifically for the generation of SAW on non-piezoelectric surfaces, several different arrangements have already been proposed, which however are not suitable for focusing the SAW.
In Appl. Phys. Lett. 42, pages 411-413 (1983), a process for the generation of convergent SAW on the surface to be investigated is described, which makes use of an acoustic lens of the type mentioned in the introduction, with which however the acoustic transducer is constructed as a semicircular surface. In the defocused condition, this lens generates SAW which are focused at a point on the optical axis of the acoustic lens. More accurate investigations have, in this connection, shown that the acoustic energy converted into SAW originates only from a very narrow annular region of the irradiation surface of the acoustic lens, in respect of which region the already mentioned Rayleigh angle is maintained with regard to the inclination of the radiation. The remaining energy of the irradiated acoustic wavefield is specularly reflected at the surface of the object.
Another process by which this disadvantage is intended to be avoided is described in J. Appl. Phys. 55 (January 1984), pages 75-79. An acoustic lens having a cylindrical exit surface is inclined by its longitudinal axis relative to the surface of the object in such a manner that the axis of radiation maintains the Rayleigh angle. By this means, an improved conversion ratio of the irradiated ultrasonic wavefield into SAW is indeed achieved, but, in this case also, not all irradiated waves are in fact inclined at the Rayleigh angle, and instead of a point focus there is a line focus, over which the SAW energy is distributed.